A study of the kinetics of HTLV-1 gene expression in cultured PBMCs isolated from ATLL and HAM/TSP patients, using splice-site specific quantitative RT-PCR analysis, revealed two-phase kinetics in the ATLL cells where the levels of pX-tax/rex mRNA were inversely correlated with the expression of the other pX-orfII, pX-orfI, and hbz-sp1 transcripts [61]. The gene discussed is SP1; the disease is tropical spastic paraparesis.